This invention relates to static power frequency changers in general, and more particular to unrestricted frequency changers (UFC) which are suitable for use in adjustable speed AC motor drives.
Unrestricted frequency changers are well known in the prior art. These circuits perform direct AC to AC power conversion by constructing a three phase set of desired AC voltage waveforms with chosen magnitude and frequency that can be either higher or lower than, or equal to that of the AC power source. Synthesis of the desired three phase output is made by the selection of segments of successive voltage waves of the AC source. A typical six pulse, pulse width modulated, force commutated UFC cycloconverter is illustraetd in FIG. 1 of U.S. Pat. No. 4,488,216 issued Dec. 11, 1984 to Gyugyi et al. Such circuits require AC switching elements which are capable of blocking voltage in either direction or conducting current in either direction.
The frequency spectrum of a UFC output waveform has been shown to be independent of the amplitude of the wanted fundamental component. Furthermore, the frequencies of unwanted (harmonic) components in the output waveform are widely separated from the fundamental frequency over the total output frequency range, being more widely separated as the output frequency is decreased. Bi-directionality of the AC switching elements gives bidirectional power flow capability to the UFC so that there is inherent ability to return motor overhauling energy back to the supply. Also, the UFC transforms the lagging power factor of an induction motor into a leading power factor load reflected to the AC source.
A six pulse UFC motor drive is usually configured around a six lead motor to obviate the need for a power transformer. However, power transformers may be required where: it is necessary to transform the output voltage; isolation in required for personnel safety or emergency operation with multiple ground faults; three terminal output is required; or a twelve pulse output is required for improved freuqency spectrum characteristics or higher power rating.
The 18 AC bidirectional switches of the circuit of FIG. 1 in U.S. Pat. No. 4,488,216 may be reduced to six if an isolation transformer is introduced and if each AC switch is configured in a three arrangement to open or close star points of secondary WYE windings of the isolation transformers in an appropriate switching sequence. Such circuits are referred to as being of the NETZTAKTUMRICHTER (NTU) type, and have been proposed by Von A. Brandt in "Der Netztaktumrichter," Bulletin des Schiveizerischen Elektrotechnischen Vereins, pp. 714-727, 62 (1971) 15, 25. Juli, Luzern. NTU circuits operate identically to unrestricted frequency changers in all respects except one. Individual phase voltage control can be accomplished by the UFC but the NTU can only control the three phase voltages in a balanced fashion.